The present invention relates to methods and arrangements in a user equipment (UE) and in a radio access network of a cellular mobile network. An example of such a radio access network is the UMTS terrestrial radio access network (UTRAN). The UTRAN is illustrated in FIG. 1 and comprises at least one Radio Network System 100 connected to the Core Network (CN) 200. The CN is connectable to other networks such as the Internet, other mobile networks e.g. GSM systems and fixed telephony networks. The RNS 100 comprises at least one Radio Network Controller 110. Furthermore, the respective RNC 110 controls a plurality of Node-Bs 120,130 also referred to as radio base stations. The Node Bs are connected to the RNC by means of the Iub interface 140. Each Node B covers one or more cells and is arranged to serve the User Equipment (UE) 300 within said cell. Finally, the UE 300, also referred to as mobile terminal, is connected to one or more Node Bs over the Wideband Code Division Multiple Access (WCDMA) based radio interface 150. The network of FIG. 1 is also referred to as a WCDMA network and is based on the WCDMA standard specified by the 3:rd Generation Partnership Project (3GPP).
For cellular networks that support full mobility, it is a particular challenge to support fast, seamless and lossless cell changes. This is true both for already deployed systems, e.g. GSM/GPRS, WCDMA, CDMA2000, and also for future systems, e.g. those as being referred to as 3GPP UTRAN Long Term Evolution (LTE) or 4G-systems. “Seamless” means here that the transmission is continuous, i.e. that there is no break in the transmission during the handover (cell change). “Lossless” means that no packets are lost during the handover. A further challenge is to avoid transmission of packet duplicates caused by the handover.
Requirements for mobile data access are increasing and demand for bandwidth is growing. To meet these needs the High Speed Data Packet Access (HSDPA) specification has been defined. HSDPA is based on WCDMA evolution standardized as part of 3GPP Release 5 WCDMA specifications. HSDPA is a packet-based data service in WCDMA downlink with data transmission peak rate up to 14.4 Mbps over a 5 MHz bandwidth. Thus HSDPA improves system capacity and increases user data rates in the downlink direction. The improved performance is based on adaptive modulation and coding, a fast scheduling function and fast retransmissions with soft combining and incremental redundancy. HSDPA utilizes a transport channel named the High Speed Downlink Shared Channel (HS-DSCH) that makes efficient use of valuable radio frequency resources and takes bursty packet data into account. This is a shared transport channel which means that resources, such as channelization codes, transmission power and infra structure hardware, is shared between several users.
With HS-DSCH, there is a new HARQ retransmission layer defined in the Node B. HARQ is a fast and resource-efficient method for combating transmission errors. However, this new HARQ layer means that buffering takes place in the radio base station, making thus seamless and lossless handovers a particular challenge. It is expected that the outcome from the 3GPP UTRAN LTE work will also include a realization with a fast HARQ and a scheduler residing in the radio base station.
In short, a typical HS-DSCH handover procedure is now explained. At times of handover, the controlling node, i.e. the RNC in a WCDMA system, assigns a certain time-offset (activation time), so that the involved radio base stations, i.e. the Node Bs in a WCDMA system, and the user equipments (UE) can prepare for the handover. These preparations include receiving necessary control information, but also the transmission of any remaining data in the buffer of the “old” radio base station, i.e. the radio base station that the UE is connected to before the handover. When the time-offset has elapsed, the handover is executed. After the handover, the scheduler in the “new” radio base station, i.e. the radio base station that the UE is connected to after the handover is performed, is responsible for assigning grants and scheduling packets. Any data remaining in the “old” radio base station is discarded and possibly recovered by some outer-layer ARQ, e.g. RLC terminated in the RNC. The HS-DSCH handover is further described in the specification TS 25.931 issued by the 3GPP.
A drawback with the above described handover procedure, is that it is very difficult to assign a suitable time-offset (activation time). A suitable time-offset is required in order to be able to successfully transmit all the packets that are forwarded via the “old” radio base station, e.g. a Node B1. Furthermore, it is also desired that the transmission is seamlessly continued from the “new” radio base station, e.g. Node B2.
Turning now to FIG. 2 illustrating the problem with handover activation time. FIG. 2 illustrates a UE that is connected to Node B1 and is about to perform a handover to Node B2. Each Node B comprises a buffer for buffering incoming packets from the controlling node RNC. Packets #1 to #5 have been forwarded to Node B1, which is currently responsible for transmitting to the UE. However, the RNC does not have precise information whether the packets have been delivered to the UE or not. In the present example it is assumed that packets #3, #4, and #5 still reside in the Node B1 buffer (or are delayed in the transport network) and queuing for transmission to the UE.
For the example shown in FIG. 2, when supposing that the RNC now executes a handover to Node B2 with a certain time-offset, packet #6 and onwards are then routed to Node B2 awaiting the handover to be properly executed. However, if the offset is too low, some (or parts of) packets #3, #4, and #5 may not get enough time for transmission, and will be discarded resulting in losses and if the offset is too high, packets #3, #4, and #5 may be transmitted well on time but there may then be a discontinuity in the transmission, as packet #6 cannot be transmitted before the execution of the handover.